D-amino acids have been considered as unnatural amino acids and it has been the common belief that D-amino acids are not present in eukaryotes. However, improvements and developments of analytic methods have shown the D-amino acids are present in a considerable amount of eukaryotes and even in humans. Some of them have been shown to have physiological functions. In this book, all aspects of D-amino acid research are described: analytic methods for D-amino acids, the presence of various D-amino acids in a wide variety of organisms, nutritional aspects of D-amino acids, anabolic and catabolic enzymes for D-amino acids, physiological significance of D-amino acids, pathology of D-amino acids, industrial aspect of D-amino acids etc.